1. Field of the Invention
The present invention relates to the field of semiconductor integrated circuit (IC) manufacturing, and more specifically, to a method of using optical assist features to enhance process latitude for features printed with two mask exposures.
2. Discussion of Related Art
Integrated circuits (IC) are generally fabricated on a semiconductor substrate, such as a Silicon wafer. The process of lithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on the wafer. At each layer of the fabrication, the wafer is first coated with photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV) light. Next, the wafer is exposed to light through a mask in an exposure tool, such as a step-and-scan tool. Then the photoresist is developed to produce an image corresponding to the pattern on the mask. Next, an etch process transfers the pattern into the thin film stacks on the wafer. Finally, the photoresist is removed.
When features in a pattern become smaller than the wavelength of light used for exposure, diffraction effects will degrade the fidelity of the printed image. Consequently, advanced lithography processes often require exposure with two separate masks to print the desired patterns for a critical layer. One exposure is performed with a phase-shifting mask (PSM) to print the small features in the desired pattern. Another exposure is performed with a binary mask to remove artifacts unavoidably printed by the PSM. In addition, the binary mask also prints the large features in the desired pattern.
As the critical dimension (CD) of the features in the desired pattern continues to shrink, phase conflicts make it increasingly difficult to print all of the small features with the PSM. Consequently, some of the small features may have to be printed with the binary mask although it is not capable of as fine a resolution as the PSM.
Several options are available, but all of them are costly. One possibility is to print some of the small features on a third mask, but throughput will be reduced by the additional exposure and overlay of the masks will be difficult to control. Another possibility is to increase the numerical aperture (NA) of the step-and-scan exposure tool, thus improving resolution, but the depth-of-focus will be reduced and process latitude will be degraded. A further possibility is to improve the resolution of the photoresist, but both the exposure process and etch process will have to be re-optimized.